The long road traveled in hematopoietic stem cell gene therapy
Authors: David A. Williams
Journal: Molecular Therapy (2022)

On August 17, the US Food and Drug Administration (FDA) approved bluebird bio betibeglogene autotemcel (Zynteglo), also known as beti-cel, for the treatment of transfusion-dependent beta-thalassemia (TDT). This is the first ex vivo stem cell gene therapy product approved in the United States, capping a more than 40-year development in the use of viral vectors to modify hematopoietic stem cells for genetic therapy (Figure). TDT is caused by mutations of the beta-globin locus, which leads to decreased production of normal beta-globin, resulting in ineffective erythropoiesis in the bone marrow and a lifelong requirement for red blood cell transfusions. The transfusions extend life, but the monthly treatments and subsequent required daily chelation for attendant iron overload are a significant burden to lifestyle, have ongoing morbidities related to transfusions and viral infections, and are a lifelong financial burden. Beginning in the early 1980s, a major focus in the field was to exploit the high-efficiency gene transfer capabilities of retrovirus vectors to introduce genetic material into the genome of hematopoietic stem cells (HSC). The earliest works resulted in two seminal papers showing proof of principle in murine hematopoietic cells.1Dick J.E. Magli M.C. Huszar D. Phillips R.A. Bernstein A. Introduction of a selectable gene into primitive stem cells capable of long-term reconstitution of the hemopoietic system of W/Wv mice.Cell. 1985; 42: 71-79Abstract Full Text PDF PubMed Scopus (408) Google Scholar,2Williams D.A. Lemischka I.R. Nathan D.G. Mulligan R.C. Introduction of new genetic material into pluripotent haematopoietic stem cells of the mouse.Nature. 1984; 310: 476-480Crossref PubMed Scopus (324) Google Scholar Although there was the promise of immediate transformation of medicine by some in the field, translation to humans was slowed by difficulties in manufacturing high titer recombinant vectors on a large scale, low efficiency of transfer into human reconstituting HSCs not predicted by murine models, and leukemic events that arose in multiple γ-retrovirus trials. The switch to lentivirus vectors, use of transduction-enhancing agents and investment by biopharma in vector production methods and cell manipulation platforms has propelled successes in early phase trials in multiple rare human monogenic diseases. Many of these trials arose in academia and provided proof-of-principle demonstrations. While many early efforts focused on ultra-rare diseases, major recent efforts has been focused on less rare conditions, including the beta-hemoglobinopathies sickle cell disease (SCD) and beta-thalassemia. While the incidence of SCD in the United States is approximately 20 times that of beta-thalassemia, bluebird bio’s approach was to leverage lentivirus gene addition of a modified beta-globin sequence to increase functional hemoglobin for both diseases. The vector was based on a design utilized in an early successful academic trial3Cavazzana-Calvo M. Payen E. Negre O. Wang G. Hehir K. Fusil F. Down J. Denaro M. Brady T. Westerman K. et al.Transfusion independence and HMGA2 activation after gene therapy of human beta-thalassaemia.Nature. 2010; 467 (nature09328 [pii] 10.1038/nature09328): 318-322Crossref PubMed Scopus (1071) Google Scholar and the modification (betaA-T87Q) was intended to decrease the polymerization of sickle hemoglobin in the deoxygenated state,4Bank A. O'Neill D. Lopez R. Pulte D. Ward M. Mantha S. Richardson C. Role of intergenic human gamma-delta-globin sequences in human hemoglobin switching and reactivation of fetal hemoglobin in adult erythroid cells.Ann. N. Y. Acad. Sci. 2005; 1054: 48-54https://doi.org/10.1196/annals.1345.057Crossref PubMed Scopus (18) Google Scholar while having no apparent deleterious effect as a component of oxygen-carrying hemoglobin in thalassemia. In their pivotal trials (HGB-207 and HGB-212) for TDT presented to the FDA, which included all genotypes of TDT, 32 of 36 treated patients (89%) attained transfusion independence with a weighted untransfused average hemoglobin of 11.5 g/dL (range, 9.3–13.7 g/dL). There was an improved erythroid hyperplasia demonstrated in those patients who underwent postinfusion bone marrow examinations, although this abnormality was not completely corrected. Thus, clear efficacy was demonstrated with adverse events in these trials primarily related to the conditioning with chemotherapy and attendant side effects of pancytopenia during engraftment, hair loss, and mucositis. There was a demonstrated delay in the time to platelet engraftment compared with historical allotransplant recipients in this disease in patients receiving Eli-cel of limited clinical consequence and more pronounced in individuals with intact splenic function. A stated concern of the FDA during the Advisory Committee proceedings considering the BLA for betibeglogene autotemcel was the occurrences of myelodysplastic syndrome (MDS) and acute myeloid leukemia in bluebird’s SCD trial using the same vector.5Goyal S. Tisdale J. Schmidt M. Kanter J. Jaroscak J. Whitney D. Bitter H. Gregory P.D. Parsons G. Foos M. et al.Acute myeloid leukemia case after gene therapy for sickle cell disease.N. Engl. J. Med. 2022; 386: 138-147https://doi.org/10.1056/NEJMoa2109167Crossref PubMed Scopus (53) Google Scholar In safety studies based on 63 patients with 221 patient years of follow-up, there was no evidence of MDS or emerging MDS in the treated thalassemia patients. Based on the discussions during the committee meeting and the responses and vote of the Advisory Committee, the concerns appeared to be answered with the explanation that (1) these events in the SCD trials were not due to vector-related insertional mutagenesis, and (2) there was likely a component related to underlying disease-specific factors in SCD patients, although these have not to date been clearly defined. However, it remains to be seen whether such events might occur in the future; as with any integrating vector, insertional mutations can occur. In fact, the academic trial noted above included clonal expansion due to alterations in the HMGA2 locus.3Cavazzana-Calvo M. Payen E. Negre O. Wang G. Hehir K. Fusil F. Down J. Denaro M. Brady T. Westerman K. et al.Transfusion independence and HMGA2 activation after gene therapy of human beta-thalassaemia.Nature. 2010; 467 (nature09328 [pii] 10.1038/nature09328): 318-322Crossref PubMed Scopus (1071) Google Scholar This clonal expansion diminished with time and has not led to oncogenesis. It is noteworthy that in a different disease, cerebral adrenoleukodystrophy, using a vector that incorporates a long-terminal repeat with a strong enhancer element internally, MDS has recently been reported. The LTR enhancer element has previously been implicated in oncogenesis in murine models.6Cesana D. Ranzani M. Volpin M. Bartholomae C. Duros C. Artus A. Merella S. Benedicenti F. Sergi Sergi L. Sanvito F. et al.Uncovering and dissecting the genotoxicity of self-inactivating lentiviral vectors in vivo.Mol. Ther. 2014; 22: 774-785https://doi.org/10.1038/mt.2014.3Abstract Full Text Full Text PDF PubMed Scopus (132) Google Scholar Our clinical trial in SCID-X17Hacein-Bey-Abina S. Pai S.Y. Gaspar H.B. Armant M. Berry C.C. Blanche S. Bleesing J. Blondeau J. de Boer H. Buckland K.F. et al.A modified gamma-retrovirus vector for X-linked severe combined immunodeficiency.N. Engl. J. Med. 2014; 371: 1407-1417https://doi.org/10.1056/NEJMoa1404588Crossref PubMed Scopus (315) Google Scholar using an enhancer-deleted γ-retrovirus, which has demonstrated safety with more than 10 years of follow-up, somewhat validated this implication in a disease in which previous studies had shown multiple leukemic adverse events. In this regard, while the bluebird globin vector appears safe to date in TDT the requirement for multiple integrated copies/cell to obtain maximal phenotypic correction is noteworthy. While this approval is a critical step in the evolution of the viral vector gene therapy platform to more widespread clinical use, additional studies using gene editing strategies are also promising. These include the use of post-transcriptional gene editing using a shimiR lentivirus vector, deletion of the BCL11A erythroid-specific enhancer using zinc finger nuclease, and CRISPR technology and base editing of the γ-globin locus to generate a “hereditary persistence of fetal hemoglobin” mutation. All of these seem to induce large levels of fetal hemoglobin that would predictably attenuate both beta-thalassemia and SCD phenotypes. Which of these will prove most effective and safe in the long run awaits more trial data and longer follow-up. Thus, a mechanism to fund, collect, and integrate years of safety data is urgently needed. A key component of the commercial success of these approaches may also depend on reproducibility, scalability and cost efficiency of large-scale manufacturing methods they entail. The availability of multiple effective autologous treatments for severe diseases is a laudable and truly exciting outcome of decades of research. Access to these therapies will be key to realizing the full impact on patients with terrible diseases, both in the US and globally. Bluebird has made the corporate decision to focus on approvals in the US, leaving many patients elsewhere without the option for this treatment. These technologies are likely not easily transferable to developing countries in their current formulations. In this regard, efforts to develop a scalable in vivo approach is one way to expand access to less well-resourced parts of the world. The proposed pricing of betibeglogene autotemcel of $2.8 million makes it the most expensive gene therapy to date, surpassing the price of Zolgensma, the AAV vector used to treat Spinal Muscular Atrophy (SMA). Overall, while many previous advances in treatment have benefited patients with TDT, there is a significant burden and ongoing risks of these treatments and a one-time potential cure is a major advance for patients without an appropriate donor for allotransplantation. For the field it marks another step toward the long-awaited revolution of genetic therapies.